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Recursive subtyping issue
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Date: -- (:)
From: Goswin von Brederlow <goswin-v-b@w...>
Subject: Re: [Caml-list] Recursive subtyping issue
Guillaume Yziquel <guillaume.yziquel@citycable.ch> writes:

> Andreas Rossberg a écrit :
>> On Feb 27, 2010, at 02:52, Guillaume Yziquel wrote:
>>>
>>> I've been struggling to have a type system where I could do the
>>> following subtyping:
>>>
>>> 'a t1 :> t2  and  t2 :> 'a t1
>>
>> Hm, what do you mean? Subtyping ought to be reflexive and
>> antisymmetric (although the latter is rarely proved for most type
>> systems), which means that your two inequations will hold if and
>> only if 'a t1 = t2, regardless of recursive types.
>>
>> /Andreas
...
> Not exactly, it seems.
>
> My goal is to implement a type inference barrier.
>
> You can do
>
>> type 'a q = private w
>
> and from the type inference point of view, int q and float q are two
> distinct types, that you can subtype to a common type.
>
> What I want is also to have the reverse, i.e.
>
>> type w = private 'a q
>
> But that doesn't work out this way because of the fact that 'a is unbound.

But then int q :> w :> float q and float q :> w :> int q. That would
make the whole thing somewhat pointless. Everyone could convert the type
to anything. I guess it would protect from accidentally passing the
wrong 'a q while allowing purposefully to pass any 'a q.

Why not supply conversion functions that do any additional checks to
ensure the conversion is a valid one? Consider the following:

module M : sig
  type w = Int of int | Float of float
  type 'a q = private w
  val add : 'a q -> 'a q -> 'a q
  val print : w -> unit
  val as_int : w -> int q
  val as_float : w -> float q
end = struct
  type w = Int of int | Float of float
  type 'a q = w
  let add x y = match (x, y) with
      (Int x, Int y) -> Int (x + y)
    | (Float x, Float y) -> Float (x +. y)
    | _ -> assert false(* typesystem failed us *)
  let print = function
      Int x -> print_int x
    | Float x -> print_float x
  let as_int x = match x with
      Int _ -> x
    | Float _ -> assert false
  let as_float x = match x with
      Int _ -> assert false
    | Float _ -> x
end

# let i = M.as_int (M.Int 1);;
val i : int M.q = M.Int 1

# let f = M.as_float (M.Float 1.);;
val f : float M.q = M.Float 1.

# M.add i i;;
- : int M.q = M.Int 2

# M.add i f;;
Error: This expression has type float M.q
       but an expression was expected of type int M.q

# M.print (i :> M.w);;
1- : unit = ()

MfG
        Goswin